1. Field of the Invention
The present invention relates to a digital sampling rate converter for compensating for a drop of in-band signals, and more particularly to a digital sampling rate converter for compensating for a drop of the in-band signals which is caused during a digital down-conversion or a digital up-conversion of the digital signals.
2. Description of the Related Art
In a multiband multi-mode wireless communication system, various digital signal processing techniques have been developed to process certain signals, from methods constructed by hardware to a Software-Defined Radio (SDR) technique, which is a software wireless processing technique. In such a digital signal processing technique field, studies are actively progressing with regard to RF/IF (Radio Frequency/Intermediate Frequency) signal processing, which extracts narrow-band signals from wide-band signals.
Digital down-converters (DDCs) and digital up-converters (DUCs) are programmable and capable of high-speed signal processing, thereby requiring low power and a small number of operations to complete its processing.
FIG. 1 is a block diagram showing an example of a digital down-converter employing cascaded integrator-comb (“CIC”) filters. The digital down-converter shown in FIG. 1 is a converter normally used in a receiver.
As shown in FIG. 1, the digital down-converter 10 includes a digital frequency synthesizer 16, multipliers 12 and 14, CIC decimation filters 20 and 30, sub-decimation filters 40 and 42, and down-samplers 46 and 48, and processes received signals which have been converted from analog signals into digital signals.
The respective down-sampled signals output from each of the down-samplers 46 and 48 of the digital down-converter 10 are signal-processed by a baseband signal processing unit 50 according to a predetermined processing method.
FIG. 2A is a detailed block diagram of one of the CIC decimation filters 20 or 30 shown in FIG. 1.
The CIC decimation filters 20 and 30 are filters proposed by E. B. Hogenauer, “An Economical Class of Digital Filters For Decimation and Interpolation”, IEEE Transactions on Acoustics, Speech and Signal Processing, ASSP-29, No. 2, pp. 155–162, April 1981. The CIC decimation filter does not need a multiplier, requires only a small memory, and can be programmatically controlled.
As shown in FIG. 2A, each of the CIC decimation filters 20 and 30 includes an integration module 22, a down-sampler 24, and a comb filter module 26.
The integration module 22 comprises a plurality of integrators, each of which includes an adder (22a to 23a), a delay unit (22b to 23b), and a scaler (22c to 23c).
Each of the integrators performs an integration operation with respect to an input digital signal by Equation 1.
                                          H            i                    ⁡                      (            z            )                          =                  1                      1            -                          z                              -                1                                                                        (        1        )            
where ‘z−1’ represents a delay value with respect to each of the delay units 22b and 23b. 
Therefore, an operation of the integration module 22 may be represented by Equation 2:
                                          H            I                    ⁡                      (            z            )                          =                                            (                                                1                  DM                                ⁢                                                                  ⁢                                                      H                    i                                    ⁡                                      (                    z                    )                                                              )                        N                    =                                    (                                                1                  DM                                ⁢                                                                  ⁢                                  1                                      1                    -                                          z                                              -                        1                                                                                                        )                        N                                              (        2        )            
where ‘M’ is a decimation ratio, and ‘D’ is a differential delay value of the comb filter module 26.
The down-sampler 24 performs a down-sampling operation with respect to a signal which is output from the scaler 23c accommodated in the integrator of the last stage in the integration module 22.
The comb filter module 26 comprises a plurality of comb filters, each of which includes a delay (26a to 27a) and an adder (26b to 27b).
Each of the comb filters performs a filtering operation as shown in Equation 3.Hc(z)=1−z−1  (3)
Therefore, an operation of the comb filter module 26 may be represented by Equation 4.Hc(z)=(1−z−DM)N  (4)
Accordingly, a transfer function of the whole CIC decimation filter 20 may be represented by Equation 5.
                              H          ⁡                      (            z            )                          =                                                            H                I                            ⁡                              (                z                )                                      ⁢                                                  ⁢                                          H                c                            ⁡                              (                z                )                                              =                                    (                                                1                  DM                                ⁢                                                                  ⁢                                                      1                    -                                          z                                              -                        DM                                                                                                  1                    -                                          z                                              -                        1                                                                                                        )                        N                                              (        5        )            
The ‘1/DM’, which is a scaling factor of the respective scalers 22c to 23c, is a value for compensating for a DC gain. Therefore, a frequency response spectrum of the CIC decimation filter 20 may be represented by Equation 6.
                              H          ⁢                                          ⁢                      (                          ⅇ                              j                ⁢                                                                  ⁢                ω                                      )                          =                              {                                                            sin                  ⁢                                                                          ⁢                                                            ω                      ⁢                                                                                          ⁢                      DM                                        2                                                                    DM                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                                      ω                    2                                                              ⁢                                                          ⁢                              ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                                      ω                    ⁡                                          [                                                                        (                                                      DM                            -                            1                                                    )                                                /                        2                                            ]                                                                                            }                    N                                    (        6        )            
Such a CIC decimation filter 20 does not need a multiplier, requires only a small memory, and can be controlled by software. CIC decimation filters are widely used to separate an aliasing component and an image component of a signal. However, CIC decimation filters have spectrum characteristics of the form of a sinc function. Therefore, a drop of an in-band signal causes the distortion of the signal spectrum, so that it is necessary to compensate for such drop.
FIG. 2B is a detailed block diagram showing a CIC decimation filter employed in a transmitting party.
The CIC decimation filter employed in a transmitting party, as shown in FIG. 2B, is referred to as “a CIC interpolation filter”. Also, a converter accommodated in a transmitter employing the CIC interpolation filter is referred to as “a digital up-converter (DUC)”.
Referring of FIG. 2B, the CIC interpolation filter includes a comb filter module 66, an up-sampler 64, and an integration module 62.
The comb module 66 comprises a plurality of comb filters, each of which includes a delay 66a to 67a and an adder 66b to 67b. 
The up-sampler 64 performs an up-sampling operation with respect to a signal which is output after the repeated delays and addings of the comb filter module 66.
The integration module 62 comprises a plurality of integrators, each of which includes an adder 62a to 63a, a delay 62b to 63b, and a scaler 62c to 63c. 
FIG. 3 is a graph showing a frequency response spectrum of the CIC decimation filter 20.
As shown in FIG. 3, since the CIC decimation filter 20 causes the generation of nulls at integer multiplies of 1/M, it is possible to remove an aliasing component and an image component. In addition, the stopband attenuation of the CIC decimation filter 20 has a value equal to or greater than the peak value of sidelobe.
Referring to FIG. 3, the attenuation of the aliasing or the image component is 67.8 dB, and the stopband attenuation is 51.2 dB, which represents an efficient status. The CIC decimation filter 20 exhibits desirable characteristics, such as small hardware, a small number of operations, and a requirement of a small memory capacity, but has a narrow pass band due to a wide transfer band, thereby being used in a multi-stage decimation structure.
Since a decimation ratio in the stage following the CIC decimation filter 20 is how, the drop of the in-band signals that is caused in the CIC decimation filter 20 must be compensated for.
In order to compensate for the attenuation caused by such a sinc function, designing an approximate compensation filter using a window method is generally used. In addition, studies have been made with respect to improving the attenuation caused by the sinc function using multistage CIC filters having identical characteristics. One such method is disclosed by A. Y Kwentus, Z. Jiang, and A. N. Wilson, “Application of filter sharpening to cascaded integrator-comb decimation filters”, IEEE Trans. Sig. Proc., Vol. 45, No. 2, pp. 457–467, February 1997, and a method of using an interpolated second-order linear filter is disclosed by H. J. Oh, G. C. Choi, and Y. H. Lee, “On the use of interpolated second-order polynomials for efficient filter design in programmable down conversion”, IEEE J. Selec. Areas Comm., pp. 551–560, Vol. 17, No. 4, April 1999.
To construct a compensation filter using the window method, a baseband compensation filter is used , but since the compensation filter is a twelfth or higher order filter, the filter has twelve taps or more, so that a large number of multiplication operations and addition operations are required.
By improving the attenuation using multistage CIC filters having identical characteristics, an excellent spectrum characteristic is exhibited in band and in a stopband, but the compensation operation is performed at the highest sampling rate equal to an operation clock of a CIC filter, thereby being performed by a clock of the highest frequency. The compensation operation must be performed at a high speed.
By using an interpolated second-order linear filter, a compensation function may be realized with a simple second-order structure, but a clock frequency higher than that of a baseband is required to operate the compensation filter because a compensation filter must be located at the stage directly next to a CIC filter. Also, since it is required for filters to have symmetrical structures to each other and a spectrum representing a characteristic of an interpolated filter exhibits repeated waves, it is possible to compensate for in-band attenuation, but the stopband attenuation becomes worse.